This invention relates to ferroelectric smectic liquid crystal cells.
The first types of liquid crystal cell to be used as display devices employed nematic or cholesteric phases. Those that operated in field effect mode could typically be operated with signal strengths of a few volts, but when the exciting field was removed, the liquid crystal always relaxed back into the same state within a short period of time. Then our UK Patent Specification No. 1557199 described how an electrically addressable non-volatile liquid crystal display can be provided. This employed a smectic A liquid crystal filling to provide bistability of operation. A drawback of this approach was that it required a significantly greater drive voltage. Later, in a paper entitled `Submicrosecond bistable electro-optic switching in liquid crystals` by N. A. Clark and S. T. Lagerwall appearing in Applied Physics Letters Vol.36 No. 11 pp 889-901 (June, 1980), a bistable cell switchable with smaller voltages was described that employed a ferroelectric smectic C liquid crystal filling.
The terms `bistable` and `bistability` are used here and elsewhere in this specification in relation to a situation in which a liquid crystal is electrically switchable between two latching states that are optically distinct on a macroscopic scale so that under appropriate illumination conditions, for instance by direct viewing or by viewing in position between appropriately oriented crossed polarisers, the cell is cable of functioning as a display element electrically switchable between two latched conditions of contrasting appearance.
In order to exhibit ferroelectricity, a smectic material must not only exist in an tilted smectic state such as Smectic C, I or F, but it must also be constituted by a material that is intrinsically chiral, or it must include a chiral constituent to provide chirality. For a definition of the ordering of the different smectic phases, reference may be made to the book entitled `Smectic Liquid Crystals Textures and Structures` by G. W. Gray and J. W. Goodby, published by Leonard Hill (1984), and in particular to the diagrams appearing on page 153 of that book. According to convention chirality may be signified by an *, and thus the material employed in the Clark and Lagerwall cell may be described as a C* material.
The chirality of a ferroelectric liquid crystal material in a C*, I* or F* phase means that its molecules have a natural tendency to align themselves in progressively different directions in succeeding smectic layers. If the layers are arranged in parallel planes this progression defines a helix, and the pitch of this helix is typically in the region of 2 to 3 microns unless it has been lengthened by diluting the chiral molecules with non-chiral ones or with further chiral molecules of the opposite handedness.
The Clark and Lagerwall paper previously referred to describe the bistable operation of a cell with a 1.5 micron thick layer of DOBAMBC or HOBACPC maintained in a C* phase with its smectic layers aligned in parallel planes perpendicular to the plane of the liquid crystal layer itself. Under these conditions it was observed that the tendency to helical arrangement of the liquid crystal molecules had been suppressed, and the authors attributed the bistable operation they found to this suppression of the helical structure by surface stabilisation.
In a later paper entitled `Ferroelectric Liquid Crystal Electro-Optics Using the Surface Stabilised Structure` appearing in Mol. Cryst. Liq. Cryst. 1983 Vol. 94 pp 213-234 these authors, in collaboration with M. A. Handschy, report further about work undertaken with such cells, describes the bistability observed in a cell containing a 2 micron thick layer of C* phase DOBAMBC, and in both C* and I* HOBACPC in a cell 1.5 microns thick cell. (In this paper the I* phase of HOBACPC has been incorrectly identified as the F* phase, but this has been corrected in later publications.)
The validity of the theory that the suppression of helix formation is effective in providing bistability of operation has been investigated by ourselves and others. In our work with C* phase material we have been unable to demonstrate any bistability of operation in cells providing a liquid crystal layer thickness of 4 microns or greater even when the pitch of the bulk material filling the cell was several times greater than the layer thickness. This finding is given support in the literature. Thus P. E. Cladis and H. R. Brand, in a paper entitled `"Soliton switch" in chiral smectic liquid crystals`, appearing in Physical Review A Vol. 28 No. 1 pp 512-4 (July 1983), report that in their investigations performed using 10 microns thick cells filled with C* phase materials having pitches in the range from 10 to 100 microns they found no evidence of bistability even with switching fields as high as 6 x 10.sup..sup.5 V cm.sup.-1. This paper concludes with the sentence, `Furthermore, we have demonstrated that, in general, production of samples with a thickness smaller than the pitch does not lead to bistability in chiral smectics`. Similarly in a paper entitled `Synthesis, Properties and Applications of Ferroelectric Smectic Liquid Crystals` appearing in Ferroelectrics, 1983, Vol. 49 pp 275 to 284, in the section entitled `Applications`, J. W. Goodby states that, `The smectic phases C*, I* and F* can be used in three different ways. (1) Thin cells, 1-3 microns thick in which the helix is unwound and the cell is bistable. The switching speed from one tilt domain to another is in the microsecond range. Optical contrast is achieved with crossed polarisers. (2) Thick cells where the helix is unwound. The cell is not bistable but the switching speed can still be in the microsecond range. This cell has similar viewing angle properties and contrast to a conventional twisting nematic device. (3) Thick cells where the helix is not unwound. The cell is not bistable and has similar properties to (2)`.
In summary, for C* phase material of any pitch, no bistability of operation has been reported for cells having a liquid crystal layer thickness of greater than 3 microns. Similarly no prior art reports any other type of ferroelectric cell with a liquid crystal layer thickness greater than 3 microns that exhibits bistability of operation. It is believed that, for pitches up at least to several tens of microns, the pitch is unwound when the layer thickness is less than the pitch. It therefore appears that the suppression of pitch is not the paramount factor determining whether or not bistability of operation is exhibited. We believe that bistability is determined by the type of order present in the phase of the material present in the cell.